PERTH, Australia — Australia’s prime minister said Friday that authorities are confident that a series of underwater signals detected in a remote patch of the Indian Ocean are coming from the missing Malaysia Airlines plane.

Tony Abbott told reporters in Shanghai, China, that search crews had significantly narrowed down the area they were hunting for the source of the sounds, first detected on Saturday.

“We have very much narrowed down the search area and we are very confident that the signals that we are detecting are from the black box on MH370,” Abbott said.

“Nevertheless, we’re getting into the stage where the signal from what we are very confident is the black box is starting to fade,” he added. “We are hoping to get as much information as we can before the signal finally expires.”

The plane’s black boxes, or flight data and cockpit voice recorders, could help solve the mystery of why Flight 370 veered so far off course when it vanished on March 8 on a trip from Kuala Lumpur, Malaysia, to Beijing. But the batteries powering their locator beacons last only about a month — and it has been more than a month since the plane disappeared.

The Australian ship Ocean Shield, which is towing a U.S. Navy device to detect signals emanating from the beacons on a plane’s flight data and cockpit voice recorders, first picked up two underwater sounds on Saturday that were later determined to be consistent with the pings emitted from the flight recorders, or “black boxes.” The ship’s equipment detected two more sounds in the same general area on Tuesday.

“We are confident that we know the position of the black box flight recorder to within some kilometers, but confidence in the approximate position of the black box is not the same as recovering wreckage from almost 4 1/2 kilometers beneath the sea or finally determining all that happened on that flight,” Abbott said.

An Australian air force P-3 Orion, which has been dropping sonar buoys into the water near where four sounds were heard earlier, picked up another “possible signal” on Thursday, but Angus Houston, who is coordinating the search for Flight 370 off Australia’s west coast, said in a statement that an initial assessment of the signal had determined it was not related to an aircraft black box.

Houston said the Ocean Shield was continuing on Friday to use its towed pinger locator to try and locate additional signals. The underwater search zone is currently a 1,300-square-kilometer (500-square-mile) patch of the ocean floor, about the size of the city of Los Angeles.

“It is vital to glean as much information as possible while the batteries on the underwater locator beacons may still be active,” Houston said in a statement. “The AP-3C Orions continue their acoustic search, working in conjunction with Ocean Shield, with three more missions planned for today.”

The Bluefin 21 submersible takes six times longer to cover the same area as the ping locator being towed by the Ocean Shield and would take six weeks to two months to canvass the current underwater search zone.

“On the information I have available to me, there has been no major breakthrough in the search for MH370,” he added. “I will provide a further update if, and when, further information becomes available.”

The searchers are trying to pinpoint the location of the source of the signals so they can send down a robotic submersible to look for wreckage and the flight recorders from the Malaysian jet. Houston said on Friday that that decision could be “some days away.”

Houston’s coordination center said the area to be searched for floating debris on Friday had been narrowed to 46,713 square kilometers (18,036 square miles) of ocean extending from 2,300 kilometers (1,400 miles) northwest of Perth. Up to 15 planes and 13 ships would join Friday’s search.

Thursday’s search of a 57,900 square kilometer (22,300 square mile) area of ocean in a similar location reported no sightings of potential wreckage, the center said.

The sonar buoys are being dropped by the Australian air force to maximize the sound-detectors operating in the search zone. Royal Australian Navy Commodore Peter Leavy said each buoy is dangling a hydrophone listening device about 300 meters (1,000 feet) below the surface and transmits its data via radio back to a search plane.

Houston has expressed optimism about the sounds detected earlier in the week, saying Wednesday that he was hopeful crews would find the aircraft — or what’s left of it — in the “not-too-distant future.”

Separately, a Malaysian government official said Thursday that investigators have concluded the pilot spoke the last words to air traffic control, “Good night, Malaysian three-seven-zero,” and that his voice had no signs of duress. A re-examination of the last communication from the cockpit was initiated after authorities last week reversed their initial statement that the co-pilot was speaking different words.

The senior government official spoke on condition of anonymity because he wasn’t authorized to speak to the media.